The present invention relates to railway car freight brake control valve devices and particularly to a quick service limiting valve, which provides a secondary phase of quick service brake control.
The present standard ABD and ABDW freight brake control valve devices employ a two-phase quick service function, the first phase or preliminary quick service occurring upon initial movement of the service piston from release position toward application position in response to a train line reduction of brake pipe pressure. This initial movement of the service piston establishes a communication between the car brake pipe and a quick service volume that is vented to atmosphere via a quick service exhaust choke. In this manner, a local reduction of brake pipe pressure is obtained at each car of a train to supplement the train line brake pipe reduction and accordingly encourage continued movement of the service piston to application position without hesitation.
In application position of the service piston, preliminary quick service is terminated by cutting off brake pipe pressure from the quick service volume at the service piston slide valve, and a secondary phase of quick service commences. During this secondary phase of quick service, the service piston slide valve connects brake pipe pressure to the car brake cylinder concurrently with auxiliary reservoir pressure until approximately 8-12 psi brake cylinder pressure develops. The brake pipe pressure is connected to the brake cylinder via a quick service limiting valve that is biased toward application position by a control spring.
It has been found that where relatively high pressure differentials exist between brake pipe and auxiliary reservoir during the first phase preliminary quick service, the degree of service piston movement into the service zone is such that maximum flow of the auxiliary reservoir pressure is established. Such high flow of auxiliary reservoir pressure may exceed the downstream flow capacity to the brake cylinder, consequently creating a short term back pressure surge or pulse in the brake cylinder feedback passage leading to the quick service limiting valve. Consequently, the brake cylinder pressure signal at the limiting valve control piston reaches a cut-off value of 10-12 psi prior to the actual brake cylinder pressure, which causes premature closure of the limiting valve against its control spring. When this high back pressure surge dissipates following repositioning of the service graduating valve, the quick service limiting valve is intended to quickly reapply to sustain the secondary phase quick service, it being understood that this local reduction of brake pipe pressure reinforces and hastens the initial brake pipe pressure reduction throughout the train.
The quick service limiting valve employed in the aforementioned ABD and ABDW control valves has either two or three O-ring seals on the limiting valve stem to prevent leakage of pressure from the brake pipe to the brake cylinder when the limiting valve is in its cut-off position following termination of secondary quick service. The friction of these O-rings can influence operation of the limiting valve in terms of causing operating variation and valve hysteresis, which unduly delays re-application of the limiting valve following the aforementioned premature cut-off and thereby reduces the efficiency of the quick service action in assisting the brake pipe pressure reduction.